1. Field of Invention
The present invention relates to electronic inverter-type ballasts adapted for operation on ordinary electric utility power lines and operable to power fluorescent lamps.
2. Prior Art and Underlying Rationale
Two major issues associated with electronic ballasts for fluorescent lamps relate to reliability and efficiency.
Reliability has become an issue as a result of widespread field experience to the effect that most presently available electronic ballasts are either unreliable, short-lived, or both.
Efficiency is an issue in the sense that the very reason for using electronic ballasts in the first place involves the attainment of significantly improved overall efficiency--and every percentage-point of efficiency improvement is economically significant. However, in the most common of all applications, namely in powering more-or-less ordinary 48" bi-pin fluorescent lamps, efficiency improvements attained by electronic ballasts can vary from a low of under 10% to a high of over 25%--with most electronic ballasts exhibiting efficiencies well below the maximally attainable level.
Two important reasons underlying limited reliability and durability of an electronic ballast relate to: (i) the use of an electrolytic capacitor for filtering the DC voltage, thereby introducing all the life-durability problems associated with such capacitors when used in high temperature environments--especially under conditions of having to handle exceptionally large ripple currents; and (ii) the use of an inverter circuit that is overly sensitive to voltage transients, thereby giving rise to high probabilities of precipitous inverter failures.
Two factors importantly limiting the attainment of maximum efficiency relate to: (i) the use in the ballast output circuit of an isolation transformer in order to meet the required standards for shock hazard safety; and (ii) the use of unfiltered DC voltage to supply the inverter, which--although eliminating any problems with the use of electrolytic capacitors--results in a strongly amplitude modulated high frequency current being provided to the fluorescent lamp, thereby causing this lamp to operate with reduced efficiency.
Exacerbating the problem of attaining maximum efficiency is the mandatory requirement that the ballast draw power from the power line with a high power factor.